Abstract: Objective: To measure the positioning error of two mask fixation methods in radiotherapy of nasopharyngeal carcinoma(NPC).Methods: Ten patients with NPC were irradiated using mask fixation method with and without windowing mark.The film was taken every week and the stray value of the field center point was measured compared with that of the localization film.Results: The X-axis stray value of field center point using mask fixation method with windowing mark was(1.25±0.56)mm,while that without windowing mark was(2.67±1.56)mm.There was a significantly statistical difference in two groups(P<0.01).The Y-axis stray value of field center point using mask fixation method with windowing mark was(1.32±0.60)mm while that without windowing mark was(2.91±1.71)mm.There was a significantly statistical difference in two groups(P<0.05 to P<0.01).Conclusions: The reproducibility and accuracy of mask fixation method with windowing mark were better than that without windowing mark,and it is one of effective methods in order to improve the positioning precision of radiotherapy.